Method and algorithm for performing an NH3 sensor rationality diagnostic

ABSTRACT

An example method includes interpreting an NH 3  composition value at a position upstream of a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit of an engine, interpreting a NO x  composition value at a position downstream of the SCR element, and determining an NH 3  sensor rationality threshold in response to the upstream NH 3  composition value. The method further includes determining an NH 3  sensor health value as indicating a sensor failure in response to the downstream NO x  composition value exceeding the NH 3  sensor rationality threshold.

RELATED APPLICATIONS

The present application is a continuation of International Patent Application No. PCT/US2013/029675 filed on Mar. 7, 2013, which claims the benefit of the filing date of U.S. Provisional Patent Application 61/607,703 filed on Mar. 7, 2012, each entitled METHOD AND ALGORITHM FOR PERFORMING AN NH3 SENSOR RATIONALITY DIAGNOSTIC, each of which is incorporated herein by reference in its entirety for all purposes.

BACKGROUND

The technical field generally relates to selective catalytic reduction (SCR) systems having an NH₃ sensor included as a control element. A failure in the NH₃ sensor may significantly impact the control of an SCR system relying upon the NH₃ sensor. Because the system relies upon the NH₃ sensor, in certain applications a failure of the sensor must be detected, and/or the impact of the lack of a sensor failure detection must be accounted for in an estimate of the impact of emissions of the system including the sensor. Detection of a failed NH₃ sensor, including at a position downstream of a reductant injector and at least a portion of the total SCR catalyst present in the system, is challenging. Therefore, further technological developments are desirable in this area.

SUMMARY

One embodiment is a unique method for determining a failed NH₃ sensor in an SCR aftertreatment system. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system having an NH₃ sensor and a controller structured to functionally execute operations to diagnose the sensor.

FIG. 2 is a schematic diagram of a controller structured to functionally execute operations to diagnose an NH₃ sensor.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, any alterations and further modifications in the illustrated embodiments, and any further applications of the principles of the invention as illustrated therein as would normally occur to one skilled in the art to which the invention relates are contemplated herein.

Referencing FIG. 1, a system 100 includes an internal combustion engine 102 producing an exhaust stream as a byproduct of operation, an exhaust conduit 116 fluidly coupled to the engine and structured to receive the exhaust stream. The engine 102 may be any type of engine understood in the art, including a diesel, gasoline, turbine, etc., that produces NO_(x) gases during operation. The system 100 further includes a selective reduction catalyst (SCR) element 104 fluidly disposed in the exhaust conduit 116. The SCR catalyst 104 includes an upstream side (toward the engine) and a downstream side (away from the engine).

The system 100 further includes an NH₃ sensor 106 operationally coupled to the exhaust conduit 116 that provides an upstream side NH₃ composition value. Without limitation, the NH3 sensor 106 may be utilized in controls of the system 100, for example to provide feedback to a controller 110 for operating a reductant injector 120 that injects a reductant 118 into the exhaust stream 116. The example system 100 further includes a NO_(x) sensor 108 operationally coupled to the exhaust conduit 116 at a position downstream of the SCR element 104, where the NO_(x) sensor 108 provides a downstream side NO_(x) composition value.

In certain embodiments, the system 100 further includes a controller 110 (labeled ECM—electronic control module, in the example of FIG. 1) structured to perform certain operations to diagnose the NH₃ sensor 106. In certain embodiments, the controller 110 forms a portion of a processing subsystem including one or more computing devices having memory, processing, and communication hardware. The controller 110 may be a single device or a distributed device, and the functions of the controller 110 may be performed by hardware or software.

In certain embodiments, the controller 110 includes one or more modules structured to functionally execute the operations of the controller 110. In certain embodiments, the controller 110 includes a composition module, an NH₃ sensor threshold module, and an NH₃ sensor diagnostic module. More specific descriptions of certain embodiments of controller operations are included in the section referencing FIG. 2.

An example system 100 further includes a second SCR element 112 disposed in the exhaust conduit 116 at a position upstream of the NH₃ sensor 106, and/or an ammonia oxidation catalyst 114 (AMOX) disposed in the exhaust conduit 116 at a position downstream of the NH₃ sensor 106 and upstream of the NO_(x) sensor 108. The first and second SCR elements 104, 112 may be two distinct catalyst “bricks” (substrate units) or a shared brick with a break or uncatalyzed zone therein, and the NH₃ sensor 106 is positioned therebetween. In certain embodiments, the first and second SCR elements 104, 112 may be positioned within the same outer housing or in distinct housings. The AMOX 114, when present, is provided with an oxidizing catalyst component that can oxidize a portion of any remaining reductant that slips past the second SCR element 104, for example to prevent excess ammonia from being emitted to the environment.

FIG. 3 is a schematic illustration of a processing subsystem 300 including a controller 110. The controller 110 includes a composition module 202, an NH₃ sensor threshold module 204, and an NH₃ sensor diagnostic module 206. In certain embodiments, the controller 110 further includes a testing conditions module 208. The description herein including modules emphasizes the structural independence of the aspects of the controller, and illustrates one grouping of operations and responsibilities of the controller. Other groupings that execute similar overall operations are understood within the scope of the present application. Modules may be implemented in hardware and/or software on non-transitory computer readable medium, and modules may be distributed across various hardware or software components.

Certain operations herein are described as interpreting one or more parameters. Interpreting, as utilized herein, includes receiving values by any method known in the art, including at least receiving values from a datalink or network communication, receiving an electronic signal (e.g. a voltage, frequency, current, or PWM signal) indicative of the value, receiving a software parameter indicative of the value, reading the value from a memory location on a non-transitory computer readable medium, receiving the value as a run-time parameter by any means known in the art, and/or by receiving a value by which the interpreted parameter can be calculated, and/or by referencing a default value that is interpreted to be the parameter value.

The composition module 202 interprets the upstream NH₃ composition value 210 and the downstream NO_(x) composition value 212. The composition values 210, 212 may be determined by any method known in the art, including sensors which provide ppm values, concentrations, and/or any other representations that correlate to and that can be used to calculate the NO_(x) and/or NH₃ present. The upstream position is upstream of an SCR catalyst element, which may be the second SCR catalyst element of a pair, or any SCR catalyst element within a group of SCR catalyst elements provided in a series or parallel configuration. The NO_(x) composition value 212 is downstream of the SCR catalyst element, and in certain embodiments may be further downstream of an oxidation catalyst such as an AMOX catalyst.

In certain embodiments, the NH₃ sensor threshold module 204 determines an NH₃ sensor rationality threshold 214 in response to the upstream NH₃ composition value 210, and the NH₃ sensor diagnostic module 206 determines an NH₃ sensor health value 216 in response to the downstream NO_(x) composition value 212 and the NH₃ sensor rationality threshold 214. For example, an NH₃ sensor threshold module 204 determines the value of the upstream NH₃ composition value 210, determines the NH₃ sensor rationality threshold 214 as a function of the upstream NH₃ composition value 210, and determines the NH₃ sensor health value 216 as a positive health indicator value if the downstream NO_(x) composition value 212 is less than the NH₃ sensor rationality threshold 214 and determines NH₃ sensor health value 216 as a negative health indicator value if the downstream NO_(x) composition value 212 is greater than the NH₃ sensor rationality threshold 214.

An example system includes an SCR catalyst element, an AMOX positioned downstream of the SCR catalyst element, an NH₃ sensor positioned upstream of the SCR catalyst element, and a NO_(x) sensor positioned downstream of the ammonia oxidation catalyst. Given an ammonia concentration x at the NH₃ sensor position, a NO_(x) conversion efficiency of η₂ in the SCR catalyst element, and an NH₃ to NO_(x) (ammonia to NO_(x) ratio, ANR), it can be seen that the NO_(x) concentration downstream of the SCR catalyst element at nominal operating conditions should be:

$\begin{matrix} {{{NOx}\left( {SCR}_{downstream} \right)} = {\frac{x}{ANR}\left( {1 - \eta_{2}} \right)}} & {{Eq}.\mspace{14mu} 1} \end{matrix}$

Further, given an AMOX conversion efficiency of η₃, (moles NH₃ converted/moles NH₃ present at AMOX inlet), it can be seen that the NH₃ slipping from the AMOX should be:

$\begin{matrix} {{NH}_{3{slip}} = {\left( {x - {\frac{x}{ANR}\eta_{2}}} \right)\left( {1 - \eta_{3}} \right)}} & {{Eq}.\mspace{14mu} 2} \end{matrix}$

Accordingly, an estimate for the NO_(x) reading of the NO_(x) sensor, assuming a conservative estimate that all of the NH₃ present will be detected as NO_(x), is found in Eq. 3:

$\begin{matrix} {{{NOx}\left( {AMOX}_{downstream} \right)} = {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + {\left( {x - {\frac{x}{ANR}\eta_{2}}} \right)\left( {1 - \eta_{3}} \right)}}} & {{Eq}.\mspace{14mu} 3} \end{matrix}$

In certain embodiments, if the AMOX conversion efficiency is understood to be less than or equal to 100% (i.e. 1), then Eq. 3 can be rewritten as:

$\begin{matrix} {{{NOx}\left( {AMOX}_{downstream} \right)} \leq {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + \left( {x - {\frac{x}{ANR}\eta_{2}}} \right)}} & {{Eq}.\mspace{14mu} 4} \end{matrix}$

The value for the NO_(x) conversion efficiency (η₂) of the SCR catalyst element varies as is known in the art, and according to the design of a particular system. Values for η₂ that exceed 50% are well known, and in many instances the η₂ will be much greater than 50%. From Eq. 4, it can be seen that a value of η₂ that is greater than or equal to 50%, and an ANR of 1, provides for: NOx(AMOX_(downstream))≦x  Eq. 5

In certain embodiments, the NH₃ sensor rationality threshold 214 is set to the value x, whereupon the NH₃ sensor threshold module 204 determines the NH₃ sensor health value 216 to be a negative health indicator value (e.g.—failed) in response to the downstream NO_(x) composition value 212 exceeding the upstream NH₃ composition value 210.

An example system includes the NH₃ sensor diagnostic module determining the NH₃ sensor health value as failed in response to the equation

${{NOx}\left( {AMOX}_{downstream} \right)} > {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + {\left( {x - {\frac{x}{ANR}\eta_{2}}} \right){\left( {1 - \eta_{3}} \right).}}}$ For example, where the downstream NO_(x) sensor reading exceeds the expected NOx sensor reading based upon NH₃ present at the NH₃ sensor, with subtracted conversion of NH₃ and NO_(x) on the SCR catalyst, and added NO_(x) production on the AMOX catalyst, the NH₃ sensor is determined to be failed. The failure may be determined to be an in-range sensor failure, for example when the NH₃ sensor is reporting a value that is within the operational limits of the sensor but failed according to the equation.

An example system includes the NH3 sensor diagnostic module determining the NH3 sensor health value as failed in response to the equation

${{NOx}\left( {AMOX}_{downstream} \right)} > {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + {k\left( {x - {\frac{x}{ANR}\eta_{2}}} \right)}}$ (see Eq. 6). For example, where the downstream NO_(x) sensor reading exceeds the expected NOx sensor reading based upon NH₃ present at the NH₃ sensor, with subtracted conversion of NH₃ and NO_(x) on the SCR catalyst. It can be recognized that the equation ignoring creation of NOx on the AMOX catalyst will result in a more conservative sensor failure estimate, as the NOx sensor will tend to read a higher value due to NOx creation on the AMOX, and if ignored will more easily trigger a failure detection.

Having the benefit of the disclosures herein, it can be seen that various multiples of the “x” in Eq. 5 may be utilized as values for the NH₃ sensor rationality threshold 214. Without limitation, the presently known AMOX efficiency and/or the presently known NO_(x) conversion efficiency of the SCR catalyst element (η₃)may be utilized to determine values for the NH₃ sensor rationality threshold 214. In certain embodiments, cutoff values for the AMOX efficiency and/or the NO_(x) conversion efficiency of the SCR catalyst element may be utilized—for example it may be known with a high degree of confidence that the NO_(x) conversion efficiency of the SCR catalyst element is greater than 75%, then the downstream NO_(x) composition value 212, at an ANR of 1, should not exceed 50% of the upstream NH₃ composition value 210, and in one example the NH₃ sensor rationality threshold 214 is set to 0.5 x.

In certain embodiments, the cross-sensitivity of the NO_(x) sensor to NH₃ may be accounted for, such as:

$\begin{matrix} {{{NOx}\left( {AMOX}_{downstream} \right)} \leq {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + {k\left( {x - {\frac{x}{ANR}\eta_{2}}} \right)}}} & {{Eq}.\mspace{14mu} 6} \end{matrix}$

In Eq. 6, the k is the cross-sensitivity of the NO_(x) sensor, for example as the ratio of the mass or moles of NO_(x) read by the sensor per unit mass or mole of NH₃ present at the sensor. In certain embodiments, the presently known, estimated, or cutoff value for the ANR is utilized to determine the NH₃ sensor rationality threshold 214, for example as in Eq. 4. In certain embodiments, various modeling equations 220 may be present on the controller 110 and/or in communication with the controller 110, and the NH₃ sensor rationality threshold 214 may be determined in response to the modeling equations 220.

In certain embodiments, the NH₃ sensor diagnostic module 206 further determines the NH₃ sensor health value 216 as indicating a sensor failure 224 in response to the downstream NO_(x) composition value 212 exceeding the NH₃ sensor rationality threshold 214. In still further embodiments, the NH₃ sensor diagnostic module further determines the NH₃ sensor health value 216 as indicating an in-range failure 226 in response to the upstream NH₃ composition value 210 being an in-range value. For example, if the upstream NH₃ composition value 210 is a value within an acceptable range of NH₃ values for the NH₃ sensor, but the downstream NO_(x) composition value 212 exceeds the NH₃ sensor rationality threshold 214, the failure of the NH₃ sensor may be deemed to be an in-range failure 226.

Determining the NH₃ sensor health value 216 to be a negative health indicator value includes at least the operations of determining the NH₃ sensor health value 216 to be a sensor failure 224, an in-range failure 226, incrementing a sensor failure value, and/or setting one or more values of a set of values to indicate a failed sensor, where the set of values are averaged or otherwise aggregated to set a sensor failure indication. Determining the NH₃ sensor health value 216 to be a positive health indicator value includes at least the operations of determining the NH₃ sensor health value 216 to be a passed sensor, decrementing a sensor failure value, resetting a sensor failure value, and/or setting one or more values of a set of values to indicate a passed sensor, where the set of values are averaged or otherwise aggregated to set a sensor failure indication.

In certain embodiments, the NH₃ sensor rationality threshold 214 is the downstream NO_(x) composition value. Alternatively or additionally, the NH₃ sensor rationality threshold is a multiple of the downstream NO_(x) composition value 218, such as a value between 0.9 and 8.0. In certain embodiments, the NH₃ sensor rationality threshold 214 is a multiple of the downstream NO_(x) composition value 218, such as a multiple selected from the list of multiples including 0.4, 0.5, 0.8, 0.9, 1.0, 2.0, 2.5, 3.0, and 8.0.

An example system further includes a testing conditions module 208 that determines whether NH₃ sensor check conditions 230 are present, and the NH₃ sensor diagnostic module 206 further determines the NH₃ sensor has failed in response to the NH₃ sensor check conditions 230 being present. Example and non-limiting NH₃ sensor check conditions 230 include: an SCR catalyst element storage capacity being lower than a threshold value (such as by determining a temperature of the SCR catalyst element exceeds a threshold value), the SCR catalyst element being saturated with adsorbed NH₃, a temperature of the SCR catalyst element exceeding a threshold, and an exhaust flow value exceeding a threshold. In certain embodiments, the NH3 sensor check conditions 230 include determining that operating conditions of the system are in a region of the system operating space wherein models for η₂, η₃, and/or NH3 storage (and/or release) in the SCR catalyst element are valid.

Alternatively or additionally, the NH₃ sensor diagnostic module 206 performs a sensor failure verification operation, and determines the sensor is failed further in response to the sensor failure verification operation. In certain further embodiments, the sensor failure verification operation includes one or more operations selected from the operations consisting of: averaging a number of sensor test results, incrementing a fault counter in response to a sensor test indicating a failed sensor, decrementing a fault counter in response to a sensor test indicating a passed sensor, integrating the upstream NH₃ composition value and the downstream NO_(x) composition value over a predetermined period of time and comparing the integrated values to the NH₃ sensor rationality threshold, and modeling NH₃ storage on the SCR catalyst and accounting for the storage in the sensor failure verification operation.

The schematic flow descriptions which follow provide an illustrative embodiment of performing procedures for diagnosing an NH₃ sensor. Operations illustrated are understood to be exemplary only, and operations may be combined or divided, and added or removed, as well as re-ordered in whole or part, unless stated explicitly to the contrary herein. Certain operations illustrated may be implemented by a computer executing a computer program product on a non-transitory computer readable medium, where the computer program product comprises instructions causing the computer to execute one or more of the operations, or to issue commands to other devices to execute one or more of the operations.

An example procedure includes an operation to interpret an NH₃ composition value at a position upstream of a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit of an engine. The procedure further includes an operation to interpret a NO_(x) composition value at a position downstream of the SCR element, and an operation to determine an NH₃ sensor rationality threshold in response to the upstream NH₃ composition value. The procedure further includes an operation to determine an NH₃ sensor health value as indicating a sensor failure in response to the downstream NO_(x) composition value exceeding the NH₃ sensor rationality threshold.

An example procedure further includes an operation to determine the NH₃ sensor health value in response to determining that the SCR catalyst element temperature is within a NO_(x) conversion efficiency range. Another example procedure includes an operation to determine the NH₃ sensor health value in response to determining that an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element is within an NH₃ conversion efficiency range. An example procedure further includes determining the NH₃ sensor health value in response to determining that the SCR catalyst element temperature is greater than a storage capacity threshold temperature. Another example procedure includes an operation to estimate a stored NH₃ variation value, where the operation to determine the NH₃ sensor health value further includes compensating for the stored NH₃ variation value. In certain embodiments, the stored NH₃ variation value is the presently estimated net rate of NH₃ storage on the second SCR catalyst element.

The net rate of NH₃ storage includes any estimated NH₃ storage as well as any estimated NH₃ release. In one example, an accumulating algorithm tracks NH₃ storage capacity due to temperature changes in the second SCR catalyst element, determining that NH₃ storage is accruing in response to, for example, an increasing storage capacity, and/or estimated excess storage capacity in the presence of NH₃. In one example, the accumulating algorithm determines that NH₃ is being released in response to, for example, a decreasing storage capacity, and/or an estimated storage capacity that is lower than a presently estimated amount of adsorbed NH₃.

In certain embodiments, a procedure includes an operation to determine a NO_(x) conversion efficiency value of the SCR catalyst element, and an operation to determine the NH₃ sensor rationality threshold further in response to the NO_(x) conversion efficiency value. In certain embodiments, the procedure includes an operation to determine a NO_(x) conversion efficiency value of the SCR catalyst element and an operation to determine an NH₃ conversion efficiency value of an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element. The example procedure further includes an operation to determine the NH₃ sensor rationality threshold further in response to the NO_(x) conversion efficiency value and the NH₃ conversion efficiency value.

As is evident from the figures and text presented above, a variety of embodiments according to the present invention are contemplated.

A system includes an internal combustion engine producing an exhaust stream as a byproduct of operation, an exhaust conduit fluidly coupled to the engine and structured to receive the exhaust stream, and a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit. The SCR catalyst includes an upstream side and a downstream side. The system further includes an NH₃ sensor operationally coupled to the exhaust conduit that provides an upstream side NH₃ composition value. The system further includes a NO_(x) sensor operationally coupled to the exhaust conduit at a position downstream of the SCR element, where the NO_(x) sensor provides a downstream side NO_(x) composition value.

The system further includes a controller structured to functionally execute operations to diagnose the NH3 sensor. The example controller includes a number of modules that perform operations to diagnose the sensor. The example controller includes a composition module, an NH3 sensor threshold module, and an NH3 sensor diagnostic module. The composition module interprets the upstream NH₃ composition value and the downstream NO_(x) composition value, the NH₃ sensor threshold module determines an NH₃ sensor rationality threshold in response to the upstream NH₃ composition value, and the NH₃ sensor diagnostic module determines an NH₃ sensor health value in response to the downstream NO_(x) composition value and the NH₃ sensor rationality threshold.

In certain embodiments, the NH₃ sensor diagnostic module further determines the NH₃ sensor health value as indicating a sensor failure in response to the downstream NO_(x) composition value exceeding the NH₃ sensor rationality threshold. In still further embodiments, the NH₃ sensor diagnostic module further determines the NH₃ sensor health value as indicating an in-range failure in response to the upstream NH₃ composition value being an in-range value.

An example system further includes a second SCR element disposed in the exhaust conduit at a position upstream of the NH₃ sensor, and/or an ammonia oxidation catalyst disposed in the exhaust conduit at a position downstream of the NH₃ sensor and upstream of the NO_(x) sensor. In certain further embodiments, the system includes a second SCR element disposed in the exhaust conduit at a position upstream of the NH₃ sensor. In certain embodiments, the NH₃ sensor rationality threshold is the downstream NO_(x) composition value. Alternatively or additionally, the NH₃ sensor rationality threshold is a multiple of the downstream NO_(x) composition value, such as a value between 0.9 and 8.0. In certain embodiments, the NH₃ sensor rationality threshold is a multiple of the downstream NO_(x) composition value, such as a multiple selected from the list of multiples including 0.4, 0.5, 0.8, 0.9, 1.0, 2.0, 2.5, 3.0, and 8.0.

An example system further includes a testing conditions module that determines whether NH₃ sensor check conditions are present, and the NH₃ sensor diagnostic module further determines the NH₃ sensor has failed in response to the NH₃ sensor check conditions being present. In certain further embodiments, the NH₃ sensor check conditions include one or more conditions selected from the following condititions: an SCR catalyst element storage capacity being lower than a threshold value, the SCR catalyst element being saturated with adsorbed NH₃, a temperature of the SCR catalyst element exceeding a threshold, and an exhaust flow value exceeding a threshold. Alternatively or additionally, the NH₃ sensor diagnostic module performs a sensor failure verification operation, and determines the sensor is failed further in response to the sensor failure verification operation. In certain further embodiments, the sensor failure verification operation includes one or more operations selected from the operations consisting of: averaging a number of sensor test results, incrementing a fault counter in response to a sensor test indicating a failed sensor, decrementing a fault counter in response to a sensor test indicating a passed sensor, integrating the upstream NH₃ composition value and the downstream NO_(x) composition value over a predetermined period of time and comparing the integrated values to the NH₃ sensor rationality threshold, and modeling NH₃ storage on the SCR catalyst and accounting for the storage in the sensor failure verification operation.

Another example set of embodiments is a method including interpreting an NH₃ composition value at a position upstream of a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit of an engine, interpreting a NO_(x) composition value at a position downstream of the SCR element, determining an NH₃ sensor rationality threshold in response to the upstream NH₃ composition value, and determining an NH₃ sensor health value as indicating a sensor failure in response to the downstream NO_(x) composition value exceeding the NH₃ sensor rationality threshold. Certain additional or alternative operations for embodiments of the example method are described following.

An example method includes determining the NH₃ sensor health value in response to determining that the SCR catalyst element temperature is within a NO_(x) conversion efficiency range. Another example method includes determining the NH₃ sensor health value in response to determining that an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element is within an NH₃ conversion efficiency range. An example method includes determining the NH₃ sensor health value in response to determining that the SCR catalyst element temperature is greater than a storage capacity threshold temperature. Another example method includes estimating a stored NH₃ variation value, where determining an NH₃ sensor health value further includes compensating for the stored NH₃ variation value.

In certain embodiments, a method includes determining a NO_(x) conversion efficiency value of the SCR catalyst element, and determining the NH₃ sensor rationality threshold further in response to the NO_(x) conversion efficiency value. In certain embodiments, the method includes determining a NO_(x) conversion efficiency value of the SCR catalyst element, determining an NH₃ conversion efficiency value of an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element, and determining the NH₃ sensor rationality threshold further in response to the NO_(x) conversion efficiency value and the NH₃ conversion efficiency value.

Another example set of embodiments is a system including an internal combustion engine producing an exhaust stream as a byproduct of operation, an exhaust conduit fluidly coupled to the engine and that receives the exhaust stream, and a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit, where the SCR catalyst includes an upstream side and a downstream side. The system further includes an NH₃ sensor operationally coupled to the exhaust conduit and that provides an upstream side NH₃ composition value, and a NO_(x) sensor operationally coupled to the exhaust conduit at a position downstream of the SCR element that provides downstream side NO_(x) composition value.

The example system further includes a means for determining an NH₃ sensor health value in response to the upstream NH₃ composition value and the downstream NO_(x) composition value. An example means for determining the NH₃ sensor health value further includes a means for determining an NH₃ sensor rationality threshold in response to the upstream NH₃ composition value. A further example means for determining the NH₃ sensor health value indicates a sensor failure in response to the downstream NO_(x) composition value exceeding the NH₃ sensor rationality threshold. In still further embodiments, the means for determining the NH₃ sensor health value further includes a means for determining that the NH₃ sensor health value indicates an in-range failure. Another example embodiment includes the NH₃ sensor rationality threshold being one or more values selected from the values consisting of: the downstream NO_(x) composition value and a multiple of the downstream NO_(x) composition value, such as a value 0.9 and 8.0. Other example multiples of the downstream NOx composition value include 0.4, 0.5, 0.8, 0.9, 1.0, 2.0, 2.5, 3.0, and 8.0.

An example system includes an ammonia oxidation catalyst disposed in the exhaust conduit at a position downstream of the NH₃ sensor and upstream of the NO_(x) sensor. In certain embodiments, the example system includes a means for determining whether NH₃ sensor check conditions are present, and a means for preventing the determining the NH₃ sensor health value in response to the NH3 sensor check conditions not being present. Additionally or alternatively, the system includes a means for performing a sensor failure verification operation.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit of the inventions are desired to be protected. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary. 

What is claimed is:
 1. A system, comprising: an internal combustion engine producing an exhaust stream as a byproduct of operation; an exhaust conduit fluidly coupled to the engine, and structured to receive the exhaust stream; a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit, the SCR catalyst having an upstream side and a downstream side; an NH₃ sensor operationally coupled to the exhaust conduit and structured to provide an upstream side NH₃ composition value; a NO_(x) sensor operationally coupled to the exhaust conduit at a position downstream of the SCR element and structured to provide a downstream side NO_(x) composition value; and a controller, comprising; a composition module structured to interpret the upstream NH₃ composition value and the downstream NO_(x) composition value; an NH₃ sensor threshold module structured to determine an NH₃ sensor rationality threshold in response to the upstream NH₃ composition value; and an NH₃ sensor diagnostic module structured to determine an NH₃ sensor health value for the NH₃ sensor in response to a comparison of the downstream NO_(x) composition value and the NH₃ sensor rationality threshold.
 2. The system of claim 1, wherein the NH₃ sensor diagnostic module is further structured to determine the NH₃ sensor health value as indicating a sensor failure in response to the downstream NO_(x) composition value exceeding the NH₃ sensor rationality threshold.
 3. The system of claim 2, wherein the NH₃ sensor diagnostic module is further structured to determine the NH₃ sensor health value as indicating an in-range failure in response to the upstream NH₃ composition value being an in-range value.
 4. The system of claim 1, further comprising a second SCR element disposed in the exhaust conduit at a position upstream of the NH₃ sensor.
 5. The system of claim 1, further comprising an ammonia oxidation catalyst (AMOX) disposed in the exhaust conduit at a position downstream of the NH₃ sensor and upstream of the NO_(x) sensor.
 6. The system of claim 5, further comprising a second SCR element disposed in the exhaust conduit at a position upstream of the NH₃ sensor.
 7. The system of claim 6, wherein the NH₃ sensor diagnostic module is Wither structured to determine the NH₃ sensor health value as failed in response to the euuation ${{{NOx}\left( {AMOX}_{downstream} \right)} > {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + {\left( {x - {\frac{x}{ANR}\eta_{2}}} \right)\left( {1 - \eta_{3}} \right)}}};$ wherein NOx(AMOX_(downstream))comprises the downstream side NO_(x) composition value, wherein x comprises the upstream side NH₃ composition value, wherein η₂ comprises a NO_(x) conversion efficiency on the SCR catalyst, wherein η₃ comprises an AMOX conversion efficiency, and wherein ANR omprises an NH₃ to NO_(x) ratio.
 8. The system of claim 7, wherein the NH₃ sensor diagnostic. module is further structured to determine the NH₃ sensor health value as failed in-range in response to the NH₃ sensor health value being failed, and the value x being within operational limits of the NH₃ sensor.
 9. The system of claim 6, wherein the NH₃ sensor diagnostic module is further structured to determine the NH3 sensor health value as failed in response to the equation ${{{NOx}\left( {AMOX}_{downstream} \right)} > {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + \left( {x - {\frac{x}{ANR}\eta_{2}}} \right)}};$ wherein NOx(AMOX_(downstream)) comprises the downstream side NOx composition value, wherein x comprises the upstream side NH₃ composition value, wherein η₂ comprises a NO_(x) conversion efficiency on the SCR catalyst, and wherein ANR comprises an NH₃ to NO_(x) ratio.
 10. The system of claim 9, wherein the NH₃ sensor diagnostic module is further structured to determine the NH₃ sensor health value as failed in-range in response to the NH₃ sensor health value being failed, and the value x being within operational limits of the NH₃ sensor.
 11. The system of claim 1, wherein the NH₃ sensor rationality threshold comprises the downstream NO_(x) composition value.
 12. The system of claim 1, wherein the NH₃ sensor rationality threshold comprises a multiple of the downstream NO_(x) composition value, wherein the multiple comprises a value between 0.9 and 8.0.
 13. The system of claim 1, wherein the NH₃ sensor diagnostic module is further structured to determine the NH₃ sensor health value as failed in response to the equation ${{{NOx}\left( {SCR}_{downstream} \right)} > {{\frac{x}{ANR}\left( {1 - \eta_{2}} \right)} + \left( {x - {\frac{x}{ANR}\eta_{2}}} \right)}};$ wherein NOx(SCR_(downstream) ) comprises the downstream side NO_(x)composition value, wherein x comprises the upstream side NH₃ composition value, wherein η₂ comprises a NOx conversion efficiency on the SCR catalyst, wherein ANR comprises an NH₃ to NO_(x) ratio, and wherein k is a cross-sensitivity of the NO_(x) sensor to NH₃.
 14. The system of claim 13, wherein the NH₃ sensor diagnostic module is further structured to determine the NH₃ sensor health value as failed in-range in response to the NH₃ sensor health value being failed, and the value x being within operational limits of the NH₃ sensor.
 15. An electronic controller with instructions on a non-transitory computer readable medium operable to execute a method, comprising: interpreting an NH₃ composition value from an output of an NH₃ sensor at a position upstream of a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit of an engine; interpreting a NO_(x) composition value at a position downstream of the SCR element; determining an NH₃ sensor rationality threshold in response to the upstream NH₃ composition value; and determining an NH₃ sensor health value for the NH₃ sensor as indicating a sensor failure in response to the downstream NO_(x) composition value exceeding the NH₃ sensor rationality threshold.
 16. The method of claim 15, further comprising determining the NH₃ sensor health value in response to determining that the SCR catalyst element temperature is within a NOx conversion efficiency range.
 17. The method of claim 15, further comprising determining the NH₃ sensor health value in response to determining that an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element is within an NH₃ conversion efficiency range.
 18. The method of claim 15, further comprising determining the NH₃ sensor health value in response to determining that the SCR catalyst element temperature is greater than a storage capacity threshold temperature.
 19. The method of claim 15, further comprising estimating a stored NH₃ variation value, and wherein the determining an NH₃ sensor health value further comprises compensating for the stored NH₃ variation value.
 20. The method of claim 15, further comprising determining a NO_(x) conversion efficiency value of the SCR catalyst element, and wherein determining the NH₃ sensor rationality threshold is further in response to the NO_(x) conversion efficiency value.
 21. The method of claim 15, further comprising determining a NO_(x) conversion efficiency value of the SCR catalyst element, determining an NH₃ conversion efficiency value of an ammonia oxidation catalyst (AMOX) positioned downstream of the SCR element, and wherein determining the NH₃ sensor rationality threshold is further in response to the NO_(x) conversion efficiency value and the NH₃ conversion efficiency value.
 22. A system for diagnosing a sensor in an exhaust conduit fluidly coupled to an internal combustion engine operable to produce an exhaust stream as a byproduct of operation, the system comprising: a selective reduction catalyst (SCR) element fluidly disposed in the exhaust conduit, the SCR catalyst having an upstream side and a downstream side; an NH₃ sensor operationally coupled to the exhaust conduit and structured to provide an upstream side NH₃ composition value; a NO_(x) sensor operationally coupled to the exhaust conduit at a position downstream of the SCR element and structured to provide a downstream site NO_(x) composition value; and an electronic controller to determine an NH₃ sensor health value for the NH₃ sensor in response to the upstream NH₃ composition value and the downstream NO_(x) composition value.
 23. The system of claim 22, wherein the electronic controller is further configured to determine an NH₃ sensor rationality threshold in response to the upstream NH₃ composition value.
 24. The system of claim 23, wherein the electronic controller is further configured to indicate a sensor failure in response to the downstream NO_(x) composition value exceeding the NH₃ sensor rationality threshold.
 25. The system of claim 24, wherein the electronic controller is further configured to determine the NH₃ sensor health value indicates an in-range failure.
 26. The system of claim 24, wherein the NH₃ sensor rationality threshold comprises at least one value selected from the values consisting of: the downstream NO_(x) composition value, a multiple of the downstream NO_(x) composition value, wherein the multiple comprises a value between 0.9 and 8.0.
 27. The system of claim 24, wherein the multiple comprises a value selected from the values consisting of: 0.4, 0.5, 0.8, 0.9, 1.0, 2.0, 2.5, 3.0, and 8.0.
 28. The system of claim 22, further comprising an ammonia oxidation catalyst disposed in the exhaust conduit at a position downstream of the NH₃ sensor and upstream of the NO_(x) sensor.
 29. The system of claim 28, wherein the electronic controller is further configured to determine whether NH₃ sensor check conditions are present, and prevent determining the NH₃ sensor health value in response to the NH₃ sensor check conditions not being present.
 30. The system of claim 29, wherein the NH₃ sensor check conditions comprise at least one condition selected from the conditions consisting of: an SCR catalyst element storage capacity being lower than a threshold value, the SCR catalyst element being saturated with adsorbed NH₃, a temperature of the SCR catalyst element exceeding a threshold, and an exhaust flow value exceeding a threshold.
 31. The system of claim 29, wherein the controller is further configure to perform a sensor failure verification operation.
 32. The system of claim 31, wherein the sensor failure verification operation comprises at least one operation selected from the operations consisting of averaging a plurality of sensor test results, incrementing a fault counter in response to a sensor test indicating a failed sensor, decrementing a fault counter in response to a sensor test indicating a passed sensor, integrating the upstream NH₃ composition value and the downstream NO_(x) composition value over a predetermined period of time and comparing the integrated values to the NH₃ sensor rationality threshold, and modeling NH₃ storage on the SCR catalyst and accounting for the storage in the sensor failure verification operation. 